This application is based on and incorporates herein by reference Japanese Patent Applications No. 2001-256144 filed on Aug. 27, 2001 and No. 2002-154102 filed on May 28, 2002.
The present invention relates to an organic electroluminescent device, in which a plurality of organic material layers is located between an anode and a cathode. Specifically, the device includes an organic luminescent layer that emits blue light, and the device efficiently and stably emits light including the blue light as a light component.
An organic electroluminescent device (organic EL device) has relatively high luminance and wide viewing angle because of its own luminescence. In addition, the organic EL device can operate with a few to dozens of voltages, so the power unit and so on for the device can be lightened. Therefore, the organic EL device is preferably applied to flat panel displays, illumination apparatuses, backlights, and so on.
The EL device includes an anode, a cathode, and a plurality of organic material layers, which is located between the anode and the cathode. In general, the organic material layers include a hole transport layer, a luminescent layer, and a electron transport layer, and the layers are located in this order between the anode and the cathode in the direction toward the cathode. Holes are injected into the luminescent layer from the anode through the hole transport layer, and electrons are injected into the luminescent layer from the cathode through the electron transport layer. The holes and electrons recombine in the luminescent layer to generate energy, with which a luminescent material in the luminescent layer emits light.
The organic EL device of this type enables luminescence of a variety of colors by combining luminescent materials that have a different luminescent spectrum. For example, colors between green and red are provided by adding a luminescent material to another luminescent material that emits green light such as an aluminum chelate. However, there is the following problem with an organic EL device that emits light including blue light component.
The luminescent layer of the organic EL device includes a host organic material, the luminescent spectrum of which has a peak between 380 nm and 510 nm, to emit blue light. In general, the hole transport layer, which is closer to the anode than the luminescent layer is, has a relatively great energy gap. However, the luminescent layer also has a relatively great energy gap, so the energy hurdle between the layers is low enough for electrons, which are injected from the cathode into the luminescent layer, to drain into the hole transport layer.
The electrons that have drained into the hole transport layer excite a host organic material of the hole transport layer, which is made of e.g., amine-based material. The host organic material can not stably maintain the excited state, so the host organic material gradually deteriorates. As a result, the luminance of the organic EL device that emits light including blue light component is lowered at an unfavorably high rate.
A method to address the above problem is to add a guest fluorescent dye to the host organic material of the hole transport layer to prevent the excitation of the host organic material by permitting the guest fluorescent dye to emit light. For example, an organic EL device disclosed in JP-A-2000-182768, which emits light including blue light component, has a structure of this type. According to the above method, the deterioration of the host organic material of the hole transport layer is prevented to some degree. However, the method can not completely prevent the injection of electrons into the host organic material of the hole transport layer.
Alternatively, a method to prevent the injection of electrons into the host organic material of the hole transport layer is to adjust the relation in the energy gap between the organic material layers to make the luminescent layer serve as a potential well. For example, an organic EL device disclosed in JP-A-3-230583 has a structure of this type. Incidentally, it is known that color mixture luminescence is realized by forming two potential wells in an organic EL device of this type. According to the above method, it is possible to prevent the injection of electrons by trapping electrons in the potential well. However, in an organic EL device of this type, the energy gap of the luminescent layer is bound to be so narrow that the luminescent layer can not emit blue light, the energy of which is greater than the energy gap, even though the luminescent layer can emit light having wavelengths longer than that corresponding to green light.
The present invention has been made in view of the above aspects with an object to provide an organic electroluminescent device that includes an anode, a cathode, and a plurality of organic material layers including a hole transport layer, which are located between the electrodes, and efficiently and stably emits light including blue light component while a host organic material of the hole transport layer is prevented from deteriorating.
In an organic EL device according to the present invention, an organic EL device includes an anode, a cathode, a hole injection layer, a hole transport layer, an electron capture layer, a luminescent layer, and an electron transport layer. The five layers are sequentially stuck in the above order between the anode and the cathode in the direction toward the cathode. The luminescent layer includes a host organic material, the luminescent spectrum of which has a peak between 380 nm and 510 nm, and a guest fluorescent dye. The lowest energy level in the conduction band of the host organic material of the electron capture layer is lower than those of the hole transport layer and the luminescent layer, between which the electron capture layer is located.
Therefore, electrons, which are injected from the cathode into the luminescent layer and otherwise drain into the hole transport layer, are captured by the electron capture layer. Thus, the organic EL device preferably efficiently and stably emits light including blue light component while a host organic material of the hole transport layer is prevented from deteriorating.
Another organic EL device according to the present invention includes an anode, a cathode, a hole injection layer, a hole transport layer, a luminescent layer, and an electron transport layer. The four organic layers are sequentially stuck in the above order between the anode and the cathode in the direction toward the cathode. The organic EL device has no electron capture layer. Instead, the hole transport layer has the same function as the electron capture layer. That is, the hole transport layer includes a dopant, which is not fluorescent and the lowest energy level in the conduction band of which is lower than that of a host organic material of the luminescent layer. Therefore, electrons, which are injected from the cathode into the luminescent layer and drain into the hole transport layer, are captured by the dopant. Thus, the dopant prevents electrons from being injected to a host organic material of the hole transport layer, and prevents the host organic material from being excited by electrons. As a result, the organic EL device preferably efficiently and stably emits blue light while the host organic material of the hole transport layer is prevented from deteriorating
Other organic EL device according to the present invention includes an anode, a cathode, a hole injection layer, a hole transport layer, a first luminescent layer, a second luminescent layer, and an electron transport layer. The five layers are sequentially stuck in the above order between the anode and the cathode in the direction toward the cathode. Each luminescent layer includes a host organic material and a guest fluorescent dye. The host organic material of the second luminescent layer includes a first electron transport material, which transports electrons, and the luminescent spectrum of which has a peak between 380 nm and 510 nm. The host organic material of first the luminescent layer includes a hole transport material and a second electron transport material. The hole transport material transports holes, and has a luminescent spectrum that has a peak between 380 nm and 510 nm. The second electron transport material transports electrons, and the lowest energy level in the conduction band of the second electron transport material is lower than that of the first electron transport material.
Therefore, electrons injected from the cathode 40 into the first luminescent layer 33a through the second luminescent layer 33b are trapped by the second electron transport material. Electrons trapped by the second electron transport material are consumed in light emission of the guest fluorescent dye in the first luminescent layer, so electrons injected from the cathode is prevented from draining into the hole transport layer. Therefore, the host organic material of the hole transport layer is prevented from being excited by electrons. As a result, the organic EL device preferably efficiently and stably emits light including blue color while the host organic material of the hole transport layer is prevented from deteriorating.